In flameproof motors, the cables from the terminal box to the stator has to be sealed so that a possible explosion in either the stator frame or the terminal box does not traverse to other. The traditional design has attempted to overcome the problem with resin poured into the bushing from the terminal box to the stator frame.
Standards, however, evolve ever stricter and all the new generation of motors needs to fulfil the new standards. The newest standards require that the bushing is exposed to a thermal aging process, and then the bushing is tested against pressure. This has brought unforeseen problems in the design of the bushing. Resin-sealed bushings tend to fail in the test because the resin contracts during the thermal again process, opening gaps in the sealing. Hence there is need to improve the concept of the sealing.